While TBG, prealbumin and albumin constitute the universally accepted major thyroid hormone binding system in human plasma, convincing evidence for similar binding to plasma lipoproteins is lacking. In a systematic study using Sepharose CL-6B chromatography to examine plasma from normal subjects and patients with genetic lipoprotein abnormalities, the interaction between thyroid hormone and the individual lipoprotein classes (VLDL, LDL and HDL) was unequivocally demonstrated. Binding of thyroxine was greater than that of triiodothyronine, and binding appeared to depend on the protein moieties of the ipoproteins. Whether this interaction, which accounts for less than 1-2% of thyroid hormone transport, plays any physiological role requires further study. Since serum TBG levels measured by an immunoradiometric assay (IRM, Corning kit) in patients with nonthyroid illness (NTI) are lower than those measured by radioimmunoassay (RIA), the effect of lipids known to be elevated in NTI, and various drugs, were tested for their ability to inhibit the binding of thyroxine to TBG. Unsaturated fatty acids and drugs such as furosemide, diclofenac and mefenamic acid were effective inhibitors. It was concluded that RIA is preferable to IRM for measuring TBG in NTI, and that TBG-IRM is a convenient and rapid method to test for thyroid hormone binding inhibitors.